This invention is the further improvement of and based on my Adjustable Laminar Flow Restriction, U.S. Pat. No. 3,144,879 issued Aug. 18, 1964. More specifically, it is an adaptation of the Laminar Flow Restriction principal for use as automatically operated small flow control valves, as required by the process control industry and particularly by reduced scale pilot plants or laboratories.
This invention relates to a device capable of restricting the flow of liquid or gaseous media by producing a laminar flow pattern, where the potential energy of the passing fluid is gradually reduced through viscous shear friction along a very narrow opening. The efficiency of such a device depends on the ability to offer as much wetted surface to the passing fluid as possible without necessitating an increase in flow area. This can be better understood by comparing my invention with a piece of tubing. The hydraulic diameter governing the Reynolds number and consequently the amount of fluid friction created in a typical restriction may be written as EQU d=4A/U
where A is the flow area and U is the length of wetted surface surrounding the flow area in question. Then for a simple tube or orifice with A=0.785 the hydraulic diameter d=1. Assuming the identical flow area of A=0.785 and 1 as diameter of the inner flow cavity in my invention, d is then calculated to be 0.5 or only half of that of a simple orifice by providing two wetted surfaces instead of one.
Further decrease in d can be obtained by selection of a large internal diameter to flow area ratio which is not possible in orifices. Fine tapered needle valves have been used to provide laminar flow restrictions in the past, where the fluid is forced to pass between the outer wall of a tapered needle and the inner wall of a tapered orifice. However, it has been found that these valves tend to drift, that is, change their effective hydraulic diameter after some time which necessitates quite frequent recalibration. The mechanism of this drift is not completely understood but may be the result of some very minute changes in the plug position due to temperature effects or inherent mechanical stresses. It has been observed that very minute side movements of the plug will effectively change the hydraulic diameter of the valve and therefore its specific fluid resistance.
In my invention, which approaches a solid state device, any movement after initial adjustment is effectively prevented and in addition, any side movement similar to the one of a valve plug would have no effect on the hydraulic diameter as will become apparent from the following detailed description. Extensive tests showed, even after months of service, no need for recalibration of my invention, even with flow rates as low as 5 cc. per minute gas flow.
Use of two parallel surfaces as described in my previous U.S. Pat. No. 3,144,879 does indeed solve the problem of not only providing an exact and reproducible flow passage but also one that provides an extremely wide "Rangeability", i.e. the useful ratio of maximum to minimum mass flow range due to the following mathematical relationship.
If one would designate the distance between the two surfaces controlling the amount of fluid resistance as H, and the radial distance the fluid has to travel through as L, then the differential pressure necessary to pass a given mass flow M is EQU .DELTA.p=kML.upsilon./H.sup.3
wherein .upsilon. is the kinematic viscosity of the fluid and k is a dimensional constant. Thus adjusting H will change either the mass flow or the differential pressure by the third power ensuring a very wide rangeability for this device.
The above equation illustrates a dependency of mass flow to H to the third power assuming a consistant pressure drop across the valve. With a typical H or gap variation between two controlling surfaces from 0.0001" to 0.01", the controlled range of mass flow is equal to 1:100.sup.3 =1:10.sup.6 which indeed was proved to be correct through flow tests conducted on a preferred embodiment of my invention.
As can be appreciated, the task of adjusting the gap between the two controlling plate surfaces is of critical importance. Manual adjustment was solved in my previous (referenced) invention by utilizing the digressive motion of two slightly different pitched screw threads located on a common adjusting screw. This solution works fine, where manual adjustment is sufficient, but is not suitable if adjustment should be the consequence of a variation of an electronic or, preferably, pneumatic signal change from a process controlling instrument.
The present invention has overcome the problem of automatically and of minute adjustments of the controlling gap between two throttling surfaces by utilization of hydraulic amplifying means which, when interspaced between conventional linear motion type pneumatic or hydraulic actuators not only reduce motion of these actuators to the small fraction required, but in the process also amplify the force output of those conventional actuators by typically 10 to 20 times thereby effecting closure of said plates against hydrostatic pressure levels exceeding 3000 psi.
Other noteworthy objections of my invention include the provision of packless valve construction, that is, contrary to needle valves, no seals are in sliding contact with the outside means of adjustment commonly referred to as valve stem. Seals in my invention can be static types and therefore are not subject to wear regardless of the frequency of adjustment. Again in contrast to needle valves, the threaded means of adjustment may be of a special wear resisting material which does not have to be corrosion resistant and may be lubricated since it is not in contact with the fluid passing and being controlled.
Yet, still another object of my invention is the provision of a laminar flow restriction, which is rugged for long service life and which is easy and inexpensive to manufacture and which does not require matching of parts, hand honing and other special production methods heretofore required by present devices performing similar functions.
These and other objections and advantages of my invention will best be understood from the following detailed description, when considered in conjunction with the annexed drawings.